Primary open-angle glaucoma (POAG) is a progressive disease leading to optic nerve damage and, ultimately, loss of vision. The cause of this disease has been the subject of extensive studies for many years, but is still not fully understood. Glaucoma results in the neuronal degeneration of the retina and optic nerve head. Even with aggressive medical care and surgical treatment, the disease generally persists causing a gradual loss of retinal neurons (retinal ganglion cells (“RGCs”)), a decline of visual function, and ultimately blindness (Van Buskirk et al., Predicted outcome from hypotensive therapy for glaucomatous optic neuropathy, Am. J. Ophthamol., volume 25, pages 636-640 (1993); Schumer et al., The nerve of glaucoma!, Arch. Ophthalmol., volume 112, pages 37-44 (1994)).
Several theories have been proposed to elucidate the etiology of glaucoma. One theory suggests that excessive intraocular pressure (in some cases coupled with genetic defects on the optic nerve head, ROC or the optic nerve) disrupts the normal axonal transport along the optic nerve, eventually leading to RGC injury.
Disturbance of axonal transport of the optic nerve hinders traffic of intracellular molecules between the ROC cell soma and its terminal. Among the intracellular molecules of importance are neurotrophic factors. Neurotrophic factors are peptide molecules which stimulate or otherwise maintain growth of neural tissue. The transport of neurotrophic factors from the brain to the cell body of RGCs is essential to the survival of the RGCs. Deprivation of neurotrophic factors can induce apoptosis of neurons (Raff et al., Programmed cell death and the control of cell survival: lessons from the nervous system, Science, volume 262, pages 695-700 (1993)).
Deprivation of neurotrophic factors appears to be a cause of glaucoma-induced RGC apoptosis, as such causal link is supported by a great deal of experimental evidence (see, generally, Anderson et al., Effect of intraocular pressure on rapid axoplasmic transport in monkey optic nerve, Invest. Ophthalmol., volume 13, pages 771-783 (1974); Quigley et al., The dynamics and location of axonal transport blockade by acute intraocular pressure elevation in primate optic nerve, Invest. Ophthalmol., volume 15, pages 606-616 (1976); Mansour-Robaey et al., Effects of ocular injury and administration of brain-derived neurotrophic factor on survival and regrowth of axotomized retinal ganglion cells, Proc. Natl. Acad. Sci. USA, volume 91, pages 1632-1636 (1994); Meyer-Franke et al., Characterization of the signaling interactions that promote the survival and growth of developing retinal ganglion cells in culture, Neuron, volume 15, pages 805-819 (1995); and Cui et al., NT-4/5 reduces naturally occurring retinal ganglion cell death in neonatal rats, Neuroreport, volume 5, pages 1882-1884 (1994)). Such trophic factors include neurotrophins and other cytokines.
The neurotrophin (“NT”) family of peptides include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), NT-3, NT-4/5 and NT-6. They act by binding to neuron surface receptors, such as TrkA, TrkB, TrkC and p75NTR. The Trk receptors are tyrosine kinases. TrkA is selective for NGF, TrkB is selective for both BDNF and NT-4/5, whereas TrkC is selective for NT-3. After binding, the NT-receptor complex is internalized and transported via the axon to the soma. These receptors undergo ligand-induced phosphorylation and dimerization, and activate a cascade of Ras protein-mediated signal transduction events that affect multiple vital functions of the neuron (Lewin et al., Physiology of the neurotrophins, Ann. Rev. Neurosci., volume 19, pages 289-317 (1997); Segal et al., Intracellular signaling pathways activated by neurotrophic factors, Ann. Rev. Neurosci., volume 19, pages 463-489 (1996); Ebadi et al., Neurotrophins and their receptors in nerve injury and repair, Neurochem Int., volume 30, pages 347-374 (1997); Kaplan et al., Signal transduction by the neurotrophin receptors, Curr. Opin. Cell Biol., volume 9, pages 213-221 (1997)). Thus, these receptors play a fundamental role in the regulation of survival and differentiation of developing neurons and contribute to the maintenance of neuronal machinery in adult life.
In the retina, RNA of both TrkA and TrkB has been observed in RGCs, dopaminergic amacrine cells and the optic nerve (“ON”). Their expression was shown to be highly regulated during neuronal development (see, Jelsma et al., Different forms of the neurotrophin receptor trkB mRNA predominate in rat retina and optic nerve, J. Neurobiol., volume 24, pages 1207-1214 (1993); Rickman et al., Expression of the protooncogene, trk receptors in the developing rat retina, Vis. Neurosci., volume 12, pages 215-222 (1995); Ugolini et al., Trk, TrkB and p75 mRNA expression is developmentally regulated in the rat retina, Brain Res, volume 704, pages 121-124 (1995); Cellerino et al., Brain-derived neurotrophic factor/neurotrophin-4 receptor TrkB is localized on ganglion cells and dopaminergics amacrine cells in the vertebrate retina, J. Comp. Neurol., volume 386, pages 149-160 (1997)). The TrkB receptor-selective ligands, BDNF and NT-4/5, have been shown to be efficacious for the protection of RGCs. Numerous studies have shown that these NTs not only improve the survival and neurite outgrowth of ROCs in culture, but also significantly reduce axotomy-induced in vivo damage of the ON and RGCs, as well as stimulate the growth of axonal branches from regenerating RGCs (see, generally, the Anderson et al.; Quigley et al.; Mansour-Robaey et al.; Meyer-Franke et al.; and Cui et al. publications cited above). For example, a single intravitreal injection of 5 μg of BDNF prevented the death of the axotomized RGCs when administered during the first five days after injury (Mansour-Robaey et al., above). In contrast with the loss of nearly half of the axotomized RGCs in the untreated retinas, virtually all RGCs were present one week after a single injection of BDNF on Day 0. Messenger RNA expression of BDNF was significantly elevated in the rat RGC layer after ON injury (Gao et al., Elevated mRNA expression of brain-derived neurotrophic factor in retinal ganglion cell layer after optic nerve injury, Invest. Ophthalmol. Vis. Sci., volume 38, pages 1840-1847 (1997)), further suggesting the potential importance of this NT in retinal recovery.
In addition to these protective effects against mechanical damage at the retina and/or ON, neurotrophins may also be protective against other forms of neuronal insult. By a yet unknown mechanism (but possibly a suppression of the apoptosis cascade), BDNF protects CNS neurons from glutamate neurotoxicity (Lindholm et al., Brain-derived neurotrophic factor is a survival factor for cultured rat cerebellar granule neurons and protects them against glutamate-induced neurotoxicity, Eur. J. Neurosci., volume 5, pages 1455-1464 (1993)); and it has been effective in vivo in preventing ischemic cell death in the rat retina (Unoki et al., Protection of the rat retina from ischemic injury by brain-derived neurotrophic factor, ciliary neurotrophic factor, and basic fibroblast growth factor, Invest. Ophthalmol. Vis. Sci., volume 35, pages 907-915 (1994)), and hippocampus (Beck et al., Brain-derived neurotrophic factor protects against ischemic cell damage in the rat hippocampus, J. Cereb. Blood Flow Metab., volume 14, pages 689-692 (1994)).
Ciliary neurotrophic factor (CNTF) is another trophic factor that supports survival of neurons. It is part of a cytokine family structurally unrelated to neurotrophins. Both CNTF and its receptor are expressed by the Müller glia during retinal neurogenesis and differentiation (Kirsch et al., Evidence for multiple, local functions of ciliary neurotrophic factor (CNTF) in retinal development: expression of CNTF and its receptors and in vitro effects on target cells, J. Neurochem., volume 68, pages 979-990 (1997). It may also be useful in preventing glaucomatous neuropathy, since it prevents lesion-induced death of RGCs (Mey et al., Intravitreal injections of neurotrophic factors support the survival of axotomized retinal ganglion cells in adult rats in vivo, Brain Res., volume 602, pages 304-317 (1993)) and ON axonal degeneration, albeit less effective than BDNF (Weibel et al., Brain-derived neurotrophic factor (BDNP) prevents lession-induced axonal die-back in young rat optic nerve, Brain Res., volume 679, pages 249-254 (1995)).
Thus, neurotrophic factors play an ameliorative role in glaucomatous retinopathy, and retinal degeneration in general. These trophic factors, however, are peptide molecules, and are therefore difficult to exploit pharmaceutically due to bioavailability problems generally resident in the pharmaceutical administration of peptides. What are needed, therefore, are non-peptide molecules which stimulate neurotrophic activity in compromised retinal tissues, without the bioavailability problems attendant to the natural peptides.
Several neurotrophic factor stimulators have been reported in the scientific literature, for example, AIT-082 (Graul & Castaner, AIT-082, Drugs of the Future, volume 22, pages 945-947 (1997)), idebenone (Nabeshima et al., Oral administration of NGF synthesis stimulators recovers reduced brain NGF content in aged rats and cognitive dysfunction in basal-forebrain-lesioned rats, Gerontology, volume 40, supplement 2, pages 46-56 (1994)), ONO-2506 (Matsui et al., Protective effects of ONO-2506 on neurological deficits and brain infarct volume following I week of permanent occlusion of middle cerebral artery in rats, Society for Neurosci. Abstracts, volume 24, page 254 (1998)), NS521 (Gronborg et al., Neuroprotection by a novel compound, NS521, Society for Neurosci. Abstracts, volume 24, page 1551 (1998)), CB-1093 (Aimone et al., The 1α, 25(OH)2D3 analog CB-1093 induces nerve growth factor in non-human primate brain, Society for Neurosci. Abstracts, volume 24, page 292, (1998)) and Clenbuterol (Culmsee et al., NGF antisense oligonucleotide blocks protective effects of clenbuterol against glutamate-induced excitotoxicity in vitro and focal cerebral ischemia in vivo, Society for Neurosci. Abstracts, volume 24, page 295 (1998)). However, nowhere in the art has it been disclosed or suggested to use neurotrophic factor stimulators to treat glaucoma or other ophthalmic neuropathies.